Generally, edible oil which has been consumed in and discharged by restaurants, food manufacturing factories, homes, etc. (i.e., waste edible oil) is treated in a manner of, for example, burying in soil a solidified oil formed through treatment with a solidification agent; or discharging as domestic waste and burning. However, in response to the growing consciousness of clean global environment, active attempts have been made to effectively re-use such a waste oil. In one ongoing attempt, a fatty acid methyl ester is produced through transesterification with methanol, and an oil suitable for diesel fuel is produced from the ester.
Specifically, a fatty acid methyl ester produced from a vegetable oil serving as an edible oil is similar to gas oil in terms of physical properties, such as viscosity and specific gravity, and combustion properties. Therefore, feasible use of the ester as bio-diesel fuel without the requirement to modify an engine has been already discussed. Recently, the fatty acid methyl ester has been of interest as a recyclable bio-related fuel, and wide-spread utilization thereof has been started in the United States and Europe. In Europe and America, however, a fatty acid methyl ester which has been produced from fresh vegetable oil is predominantly employed. Thus, since the production cost of the ester is higher than that of gas oil, the ester is usually employed as a mixture with gas oil.
In a fatty acid methyl ester which has been produced through transesterification of a fat/oil with methanol, glycerin (by-product), monoglyceride, diglyceride (reaction intermediates), and unreacted triglyceride remain even after completion of purification. When such residues remain in a large amount, the ester fails to satisfy quality requirement for a fuel oil. Thus, the amount of the residues is preferably reduced to as low a level as possible.
In other words, in a fatty acid methyl ester single system, hydrophilic glycerin which has been solubilized in the oily fatty acid methyl ester by the mediation of monoglyceride and diglyceride serving as amphiphilic substances may be separated from the oil and precipitate as a result of changes in external factors such as storage time and temperature. In a mixture system with gas oil, monoglyceride and diglyceride are dissolved also in the gas oil, thereby reducing the solubilization degree of glycerin in oil and increasing the possibility of separation and precipitation of glycerin. If such a phenomenon occurs during storage or in a fuel piping in an automobile, the ester systems cause some problems, failing to be employed as diesel fuel. Needless to say, the amount of methanol and water remaining in the fuel must be reduced to as low a level as possible.
Therefore, in Europe and America, quality standards of fatty acid methyl esters for use as diesel fuel oil have been determined. In accordance with the standards such as Germany (DINE 51606), France (Journal official), and Italy (UNI 10635), purity of fatty acid methyl ester is 98% or more, and monoglyceride content, diglyceride content, triglyceride (unreacted) content, and glycerin content are 0.8% or less, 0.2 to 0.4% or less, 0.2 to 0.4% or less, and 0.02 to 0.05% or less, respectively. At present, European (EU) universal standards are under consideration, and a most possible candidate purity of fatty acid methyl ester is 98% or more and glycerin content of 0.03% or less, and monoglyceride content, diglyceride content, and triglyceride content are considered to be 0.8% or less, 0.4% or less, and 0.4% or less, respectively, similar to the case of the Germany standards. As stipulated in ASTM PS-121-99 (US), glycerin content is 0.02% or less. In Japan, although no particular action has been taken for setting the standards for fatty acid methyl esters for use as a fuel, the standards will be discussed in the near future with reference to the European standards or other standards, so as to prevent problems in automobiles during driving.
When fatty acid methyl ester is purified under conditions satisfying, among the aforementioned quality standards in Europe and other countries, glycerin content of 0.03% or less (first condition) and purity of fatty acid methyl ester of 98% or higher (second condition), the purified ester has satisfactory qualities (e.g., viscosity and specific gravity) for use as diesel fuel oil and reduced monoglyceride content, diglyceride content, and triglyceride content. Even though these glycerides are present in very small amounts, they may conceivably solubilize methanol, water, or other oily impurities. Accordingly, the thus-obtained fatty acid methyl ester has substantially satisfactory qualities.
In view of the foregoing, demand exists for a method for purifying an alkyl ester for the purpose of enhancing its purity and reducing glycerin content.
A method for producing fatty acid alkyl ester from a fat/oil material has conventionally been known. In one exemplified method, a fat/oil (fatty acid triglyceride) is reacted with an alcohol in the presence of acid or an alkaline substance, to thereby form a fatty acid alkyl ester, followed by purifying and washing with water so as to remove acid, an alkaline substance, and other water-soluble substances. According to the method, a mixture liquid readily forms an emulsion through washing with water, and the emulsion must be left to stand for one day so as to remove added water.
Japanese Patent Application Laid-Open (kokai) No. 7-310090 discloses an improved purification method based on washing with water, in which washing water is added to a purified mixture and the resultant mixture is heated to 70 to 90° C., thereby avoiding emulsification and accelerating phase separation. However, in order to prevent hydrolysis of the formed fatty acid methyl ester caused by high-temperature treatment, the alkaline substance employed in reaction and dissolved in the fatty acid methyl ester must be neutralized with acid. In the method, washing effect is completely attained when 20 parts by weight or more of washing water is added to 100 parts by weight of the fatty acid methyl ester, and washing is performed twice. Although the method is envisaged to lower glycerin content, the purity may be lowered through hydrolysis caused by high-temperature treatment (i.e., although the alkaline substance is neutralized by an acid substance, correct neutralization is difficult, since the reaction system contains different phases; in fact, the high-temperature treatment is performed under alkaline or acidic condition). In addition, a large amount of wastewater must be treated.
Regarding facilities that produce waste edible oil, waste edible oil is collected by small amounts from a large number of facilities, and therefore, in consideration of transfer costs and other factors (e.g., 500 to 5,000 kg-fuel/day, usually 500 to 1,000 kg-fuel/day), such waste edible oil is reasonably treated in many small-scale facilities. Thus, installation of an additional wastewater treatment facility for carrying out washing treatment with water in each small-scale facility is very disadvantageous in terms of economy and space.
Japanese Patent Application Laid-Open (kokai) No. 10-245586 discloses in detail some purification methods that do not involve washing with water, the purification methods being included in a method for producing a diesel fuel oil from a waste edible oil. The disclosed purification methods includes a purification method employing adsorption of fatty acid alkyl ester by an adsorbent such as active terra alba. Through employment of the method, no wastewater is produced, and an alkaline component employed in the reaction can be removed via adsorption. However, glycerin, monoglyceride, diglyceride, and other substances are difficult to remove, and the purified fatty acid alkyl ester has insufficient quality for serving as a diesel fuel oil.
As described above, no conventional methods for purifying fatty acid alkyl ester for diesel fuel oil satisfy qualities required of diesel fuel oil and minimization of the amounts of wastes from the production process. Particularly, minimization of the amounts of wastes is a critical issue, when “zero-emission of waste” is to be attained, to a maximum degree, in a large number of small-scale facilities employing waste edible oil as a raw material.